Separating h2s and mercaptans from hydrocarbons



Patented Mar. 8, 1938 PATENT OFFICE snrmrme H28 AND mnacAr'rANs mu nrnnocaaaons Ludwig Bosenstein,

San Francisco, cum. assignor to Shell Development Company, San Francisco, CaliL, a corporation of Delaware No Drawing-s 2Clalml.

This invention relates to the separation, by means of an absorbent agent, of volatile acids from hydrophobic liquids, i. e., liquids which are immiscible with water, and especially mineral 5 hydrocarbon oils, which may be either normally gaseous or normally liquid. By the term "volatile acids, I mean those gases which in water solution have an acid reaction, but which are released unchanged upon sufficient heating of the water. Carbon dioxide, hydrogen chloride, sulfur dioxide, mercaptans and hydrogen sulfide are the main gases of this type which are present in the hydrocarbon mixtures commonly encountered in refinery and industrial operations.

tion and recovery of these volatile acids from the liquid mixture, or may be employed to purify other liquids by the removal of said acids therefrom.

More specifically, the invention relates to the treatment of industrial liquids which contain volatile acids, particularly hydrogen sulfide, such as products from the destructive distillation of coal, and various petroleum distillates, as, for example, lubricating oils, fuel oils, kerosene, straight run or cracked gasoline, light'naphthas, or liquefied. normally gaseous hydrocarbons, etc.', with an absorbent which permits the recovery of volatile acids, particularly hydrogen sulfide, in a a concentrated form, and substantially free from hydrocarbons.

Hydrogen sulfide may be used as initial material for various end-products and it is, therefore, desirable, for my purpose, to obtain hydro- 35 gen sulfide as pure as possible. For example, if it is desired to burn hydrogen sulfide to sulfur dioxide as in the process for producing sulfuric acid, methods using organic bases or solutions thereof to separate hydrogen sulfide are objec- 40 tionable when the liquids from which His is separated contain or consist of hydrocarbons. Many of these hydrocarbons are appreciably soluble in such solutions and are sufliciently volatile to be productssuch as carbon monoxide, which are undesirable. Moreover, most of such organic bases 50 are appreciably soluble in the hydrocarbon mix- The process may be employed for the separa- Application June 10, 1935, erial No. 25.947

ture being treated and necessitate further treatment of the latter to purify the mixture and to recover the bases. The hydrogen sulfide obtained by my process may also be utilized to produce sulfur, as by oxidation in a Claus kiln, or by re- 5 acting it with sulfur dioxide produced by combustion of a portion of the hydrogen sulfide.

On boiling the solution after absorption of hy- 15 small as to be impractical. On the other hand a solution of sodium carbonate absorbs hydrogen sulphide according to the reversible reaction:

Na-.-COa+HaSz NaHCOa-}-NaHS 25 On heating the solution saturated with hydrogen sulphide, only'a portion of the hydrogen sulphide was released, the other portion remaining in the solution, thus:

so that sodium sulphide accumulated in the solution until the same condition was established as in the preceding case. I v

I have discovered that aqueous solutions of KaPO4, preferably concentrated, are suitable for my'purpose as they absorb hydrogen sulphide rapidly and evolve substantially pure hydrogen sulphide on reversing the absorption reaction as by boiling the solution at atmospheric, superatmospheric, or law absolute pressures. The reversal may be carried as far as desirable by continued boiling, and when such boiling is practiced at an elevated temperature, the regenerated solution may be cooled before returning to the absorption step. In case the solution is boiled at a low pressure and corresponding low' temperature, cooling of the solution can be dispensed with prior to its return to the absorption step.

intermittent or continuous processes.

A solution containing both mo; and mo.

may be utilized to carry out the absorption of hydrogen sulphide but I prefer to operate with a solution initially containing KsPOa only, for while KaHPO4 in the initial solution results in a higher percentage of the absorbed hydrogen sulphide being evolved for any given time of recovery, it also decreases the capacity of the solution for .hydrogen sulphide by limiting the (m) will tend to'crystallize'out in the ab-' amount of KaPO4 which can be carried' in the solution. I have found that the effective hydrogen sulphide capacity of the solution is greatest for one'which initially contains only KsPOc. Nevertheless, because of the more rapid regeneration, solutions initially containing both K1PO4 andKfliPO. may be found useful, as, for example, in'a two stage system in which the liquid is contacted in series.

Working with an aqueous solution of KsPOc, I

have found that during absorption, KaPOa is converted to KaHPO4, as shown by the reversible reaction;

1) xsrol+msa= xmroi+xns and I have found that 'this is the reaction whi takesplaceinprei'erenceto (a) zxiroi+ms=zxinrol+ms Ihave determined the desirablemaximum concentration at 25 C. to be 2.5'i mols KsPOc per 1000 g. aqueous solution. Such a solution is 88% saturated with respect to the potassium content. If a more concentrated solution is used'at this temperature, then the less soluble salt sorbing unit. Accordingly, the concentration must be adjusted with respect to the KiHPO formed during the absorption and/or added initially so that no substantial crystallisation takes place during absorption. At any temperature of operation, a too concentrated solution will deposit crystals. However, if the concentration of the solution is permitted to adjust itself by deposit ing crystals then the resulting concentration will be the desirable maximum.

I have found, contrary to all expectation, that the sodium phosphates act quite differently from potassium phosphates. First, the maximum usable concentration of sodium phosphates is much less because of their lesser'solubilities, and second, the reversal-of the absorption reaction is not nearly as rapid or complete. This is probably due to a formation of sodium sulphide in the reverse reaction, thus:

Consequently, sodium phosphate solutions are not nearly as eflicient as potassium phosphate solution, and cannot be considered as equivalents 5 thereof.

The process can be conducted at various temperatures and pressures. The contact between the hydrocarbon mixture and the absorbing liquid may be obtained by passing the two liquids into a mixing device, such as an agitator, permitting the resulting mixture to form two phases,

and separating these phases by decantation, centrifuging, or any conventional method known to the art. A series of such mixers and separators may also be employed, and may be used in multiple extraction or counter-current extraction processes. Alternatively, the two liquids may 9,110,408 The regenerated solution may be in batch,

be caused to flow countercurrently in an extraction column. To liberate the hydrogen sulfide from, the absorbing solution a regenerator of the type shown in the U. S. patent to Bottoms, 1,789,901 may be utilized. Water which is'vaporized with the hydrogen sulfide may be separated therefrom by a condenser at the top of the regenerator, as disclosed by the patent, thus maintaining the initial concentration of the absorbent agent substantially constant. If desired, water may be introduced from an outside source at suitable intervals or continuously to maintain the predetermined concentration of the absorbent.

agent if the condensate is not returned to the regenerator. Since the hydrogen sulfide leaving the system is saturated with water, the water thus removed can be compensated for by introduction from an outside source.

Mercaptans and other acidic gases may also be removed from hydrocarbons with the liquid phase potassium phosphate treatment of my invention. Potassium phosphate will scrub out hydrogen sulflde in preference to mercaptans. By limiting the quantity of the potassium phosphate solution to that approximately equivalent to the hydrogen sulfide content it is possible to recover substan-' tially pure hydrogen sulfide. The alkalinity may also be adjusted by using both H04 and KaHPOe, so that practically no mercaptan's are removed. When desired, such a treatment may be followed by an additional stage to remove the weaker acidic constituents, including mercaptans, and remaining hydrogen sulfide, if any, with a larger quantity v of scrubbing solution, and/or one of diiferent alkalinity.

I have found that certain petroleum fractions contain aliphatic acids which are absorbed by the potassium phosphate solution, but cannot be I removed therefrom by simple heating. In'such situations it is desirable to distill the hydrocarbons and treat only. the fraction which does not contain such acids, as, for example, the fraction having a C. end point. These acids may also be removed by subjecting the initial hydrocarbon mixture to a preliminary washing treatment with hydroxide.

There are. set forth hereinafter several examples of my invention which it is understood are exemplary .onLv.

Example I.A number of test samples of a liquid hydrocarbonmixture consisting of about 29.9% propylene, 69% propane, and-1.1% E18 by,

weight were each separately subjected to single batch extractions in the liquid state at room temperature with different amounts of aqueous solutions containing 2.0, 1.455 and 1.0 grams mols, of KaPOr per kilogram, respectively. The weight per cent of H118 remaining in each treated portion of the hydrocarbon mixture is given in Table I.

Table I Grams KQPOI r t solution per y N s'mpb oi hydromrmi in hm mixture w Per kilogram of Y Qu noi: KaPOr hm a o 1.45; 1. o

0 1- 1 1. 1 1. 1 0. 271 490 a 0 m 021 1D .41. 0. 250 i 010 021 l 0. 500 002 i 00: m 0.150 001 m L 000 .ml

Example II.-A number of samples of a similar Table IV hydrocarbon mixture, but containing 2.1% H28 by weight were treated with various amounts 0! Percent by weight HRS rethe potassium phosphate solutions as described Grams KsPO g a gg in y r car o above in Example I. The results are tabulated Sample Solution W in Table 11- gram olhydrocarbonmmm Liquid phase Gaseous phase treatment treatment Table II 0 2.10 2.10 0232 an 2- i b f g g H29 0750 010039 01 0086 50 11 on m Sample No. lg gg fg ggg i g 1.000 0. 0023 0. 0047 in e I These data show that scrubbing liquid hydro- Per kilogram of carbons produces decidedly better result than mols Solution 455 scrubbing the gas, especially in the situations in which a high degree of removal of H28 is desired. 0. 2.1 2.1 2.1 With a view of presenting comparable data to 3- $5 i 3 {fig show the eflect of varying the treating conditions 0 250 01430 01015 11242 and concentrations all of the above examples per- 39*; 3 8' 3, ,2 8- tain to propylene-propane mixtures. My inven- 1. 000 01002 01000 tion may, however, be applied with success to Table III Per cent by weight H18 Stage in hydrocarbon mix:

ture

Inlet 1. 1 Outlet to first plate 0. 047 Outlet to second plate 0. 0016 I have found that treating the hydrocarbon in the liquid state results in a substantial improvement over the corresponding treatment in the gaseous or vapor state. The following example demonstrates the superiority of my method over the known vapor phase treatment:

Example IV-Four samples of the hydrocarb mixture used in Example 11 (containing 2.1% H28) were separately treated in the liquid state in single batch extractions with difierent quantitles of an aqueous solution containing 1.455 mols K1PO4 per kilogram, and the percent HzS remaining in each treated sample determined. Four other samples of the same hydrocarbon mixture were treated with corresponding amounts of the same potassium phosphate solution, but in the gaseous state. The temperature and pres sure were almost the same for the treatments in the gas and in the liquid phases. The per cent by weight HaS remaining in each treated sample is shown in Table IV:

other hydrocarbons which may be normally gaseous or normally liquid. For example, light petroleum fractions, such as butane, pentane or similar light fractions, petroleum naphtha, gasoline, kerosene, as well as natural gasoline,

stabilizer bottoms, refinery recovery hydrocarbon gases and recovering gasoline may be treated in the liquid phase in accordance with the described process.

While I have in the foregoing described in some detail certain preferred embodiments of my invention and some variants thereof, it will be understood that this is only for the purpose of making the invention more clear, and that the invention is not to be regarded as limited to the details of operation described, nor is it dependent upon the soundness or accuracy of the theories which have been advanced as to the reasons for the advantageous results attained. On the other hand, the invention is to be regarded as limited only by the terms of the accompanying claims, in which it is my intention to claim all novelty inherent therein as broadly as possible in view of the prior art.-

I claim as my invention:

1. In the process of separating H28 and normally gaseous 'mercaptans from normally gaseous hydrocarbons of less than five carbon atoms by treatment with an aqueous solution of potassium phosphate, the improvement consisting of contacting said mixture while in the liquid state with the said aqueous solution containing tripotassium phosphate under conditions to maintain both, the material being treated and the treating agent, in the liquid state.

2. In the process of, separating I-IaS and normally gaseous mercaptans from normally gaseous hydrocarbons of less than five carbon atoms by treatment with an aqueous solution of potassium phosphate, the improvement consisting of contacting said mixture while in the liquid state with the said aqueous solutioninitially containing diand tri-potassium phosphate under conditions to maintain both, the material being treated and the treating agent, in the liquid state.

LUDWIG ROSENSTEIN. 

